Pollution control by oxidation of aqueous solutions or suspensions of organic waste products with an oxygen-containing gas at high temperature and pressure is a well-known procedure known as wet oxidation; see for example Zimmermann U.S. Pat. No. 2,665,249.
In prior art wet oxidation processes, waste liquor and compressed air can be directly mixed with no hazard, but if pure oxygen or an oxygen enriched gas is used to supply oxygen for the oxidation process, special precautions have to be taken to prevent a spontaneous combustion occurring at the point oxygen and waste liquor are being mixed. The primary hazard is caused by the undesirable introduction of combustible materials into the oxygen supply line, together with intermittent wetting and drying in the line such that a dry film of combustible material forms on the pipe surface and comes into direct contact with the oxygen or oxygen bearing gas. This dry combustible film ignites spontaneously, leading to fire, explosion and failure of pressurized equipment. The wetting-drying condition may be caused by an interruption in the flow or gas or liquid or even by normal variation in the flow of the gas and the liquor, or normal variations in system pressure.
In U.S. Pat. No. 4,174,280, wet oxidation of oxidizable materials which are insoluble, immiscible and difficult to suspend or emulsify in water, is accomplished by separately injecting the oxidizable materials into a sealed reactor as a separate concentrated stream. Air or oxygen is introduced into the reactor either as a separate stream or as a mixture with water or aqueous solution or suspension. Contact between oxidizable material, water and oxygen occurs in the reactor, thus preventing premature oxidation of slugs of the oxidizable material.
In U.S. Pat. No. 3,835,048, mixing of steam with pure oxygen is set forth as a safe method of increasing oxygen transfer to a waste liquor. I have found, however, that dilution of oxygen with steam is hazardous in a system in which wet oxidation occurs.
If for any reason the flow of steam is interrupted or reduced in volume, normal heat losses from the oxygen supply line result in condensation of the steam, markedly reducing its volume and creating a momentary partial vacuum in the line. Waste liquor containing oxidizable material is thus drawn into the line where spontaneous combustion will occur.
Even under normal operating conditions, small pressure surges occur in the system. Steam, being a compressible gas which condenses at pressures greater than its vapor pressure, will alternately condense and vaporize, drawing waste liquor further into the line containing oxygen as it condenses, and drying combustible solids in the line as it vaporizes, leading to dangerous spontaneous combustion.
In the following summary and in the description of the preferred embodiment of this invention, the words "pure water" or "water" will always refer to water in the liquid state.